1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) element, and further, relates to a light-emitting apparatus, an image-forming apparatus, a display apparatus, and an imaging apparatus using the organic EL element.
2. Description of the Related Art
In recent years, organic EL elements that emit light spontaneously with a low drive voltage of about several volts are drawing attention. The organic EL element has a configuration in which a reflective electrode having a metal reflective layer, a light-emitting layer, and a transparent electrode are stacked. Due to excellent features such as surface emitting characteristics, light weight, and visibility the organic EL element is being put into practical use as a light-emitting apparatus of a thin display, lighting equipment, a head-mounted display, or a light source for a printhead of an electrophotographic printer.
In particular, there is an increasing demand for low power consumption of an organic EL display apparatus, and further improvement of emission efficiency is being expected. One of element structures improving the emission efficiency remarkably is a microcavity system. Light-emitting molecules have a feature of radiating light strongly toward a space in which “enhancing interference” of light occurs. Specifically, the radiation rate of excitons can be increased and the radiation pattern thereof can be controlled through use of optical interference. According to the microcavity system, element parameters (film thickness and refractive index) are designed so that the “enhancing interference” occurs in a light-extraction direction viewed from light-emitting molecules.
In particular, it is known that, in the case where a distance d between a reflective surface of the metal reflective layer and a light-emitting position of the light-emitting layer satisfies the condition: d=1λ/(4n) (i=1, 3, 5 . . . ), a radiation intensity is increased most by an interference effect. i represents the order of interference, and hereinafter, the condition of i=1 is referred to as an interference condition of λ/4. Here, λ indicates a peak wavelength in a vacuum of a PL spectrum of light-emitting molecules, and n corresponds to an effective refractive index between a light-emitting point and a metal reflective layer. According to the microcavity system, it is not necessary to use an uneven structure such as a microlens, and an increase in the emission efficiency at low cost can be expected.
A microcavity is classified into a weak cavity and a strong cavity depending upon the magnitude of a reflectance on a light-extraction side. Generally, in the weak cavity, an electrode structure having a high transmittance such as a glass/transparent oxide semiconductor is used, and an interference effect of the cavity is determined mainly by an interference condition between a metal reflective layer and the light-emitting layer. On the other hand, in the strong cavity, a semi-transmissive metal thin film having a high reflectance is used as a transparent electrode on the light-extraction side. Therefore, the strong cavity includes not only an interference effect obtained between the metal reflective layer and the light-emitting layer but also the interference effect obtained between the light-emitting layer and a metal thin film on the light-extraction side. In this case, an optical distance between the light-emitting layer and the metal thin film is also designed so as to satisfy an interference condition of λ/4 in such a manner that the interference effect becomes maximum. Therefore, in the strong cavity, the interference effect larger than that in the weak cavity can be used, and thus, the emission efficiency can be improved remarkably.
However, it is known that, in the interference condition of λ/4, the distance between the light-emitting layer and the metal reflective layer is about 60 nm or less, compared with the interference condition of 3λ/4 (condition of i=3), and hence, a surface plasmon (SP) loss becomes particularly large. The SP loss is a phenomenon in which an SP of metal is excited by excitation energy of light-emitting molecules, and as a result, the excitation energy is transformed into Joule heat. Therefore, the microcavity using the λ/4 interference structure has a problem that the emission efficiency is not improved with respect to a large optical interference effect. Specifically, in order to further improve the emission efficiency of the microcavity under the λ/4 interference condition, a method of controlling the SP loss is required.
Hitherto, as a method of suppressing the SP loss, a method of sacrificing the interference effect of increasing the distance between the metal reflective layer and the light-emitting layer disclosed in Japanese Patent Application Laid-Open No. 2008-543074 has been proposed. Further, as shown by Jorg Frischeisen et al., Organic Electronics 12, 809-817 (2011), a method of satisfying both the interference effect of λ/4 and a suppression of the SP loss by placing a transition dipole moment of light-emitting molecules horizontally has started being proposed. The behavior of light in an organic EL element such as the SP loss can be calculated by optical simulation, and the detail thereof is found in S. Nowy et al., Journal of Applied Physics 104, 123109 (2008).
However, the above-mentioned methods of suppressing the SP loss have been studied with a weak cavity in which there is only one interface between metal and a dielectric. Specifically, the suppression of a surface plasmon in a strong cavity that satisfies the interference condition of λ/4 has not been proposed.